Transcript cis185-ROUTE-lecture4
CIS 185 CCNP ROUTE Ch. 4 Manipulating Routing Updates
Rick Graziani Cabrillo College [email protected]
Last Updated: Fall 2011
Materials
Book: Implementing Cisco IP Routing (ROUTE) Foundation Learning Guide: Foundation learning for the ROUTE 642-902 Exam By Diane Teare Book ISBN-10: 1-58705-882-0 ISBN-13: 978-1-58705-882-0 eBook ISBN-10: 0-13-255033-4 ISBN-13: 978-0-13-255033-8 2
Network Performance Issues
Common network performance issues include the following:
Excessive routing updates
: Decrease network performance CPU utilization spikes The size of the routing update The frequency of the updates
The presence of any route maps or filters
: Incorrectly configured route maps or filters can cause too much or the wrong data to be sent.
The number of routing protocols running in the same AS
: Processing the updates. Routes may also be redistributed between protocols, which can add to the number of updates that a specific protocol must process.
3
Controlling routing updates involves a variety of solutions, including the: Design changes: Limiting the number of routing protocols used Choice of routing protocol Network design (areas, stub networks, etc.) Using passive interfaces Route filtering using: Access lists Route maps Distribute lists Prefix lists 4
Route Redistribution
5
Routing protocols were not designed to interoperate with one another using different: Metrics Reactions to topology changes Timers Processes Routers using different routing protocols can exchange routing information.
Route redistribution
is the capability of
boundary routers
connecting different routing domains to exchange and advertise routing information between those routing domains. 6
Route Redistribution
One-way
route redistribution - one protocol receives the routes from another)
Two-way
route redistribution - both protocols receive routes from each other.
Boundary routers:
Routers that perform redistribution Borders two or more ASs or routing domains.
Note: The term
boundary router
is also sometimes used to describe a router running a classful routing protocol (like RIP) that has interfaces in more than one classful network.
7
Redistribution is always performed
outbound
The router doing redistribution does
not
change its routing table.
Router A
(boundary router) participates in both: OSPF EIGRP Two-way redistribution does not affect the routing table on
Router A
However:
Router C
will learn about redistributed EIGRP networks (via OSPF)
Router B
will learn about redistributed OSPF networks (via EIGRP) Only networks in
Router A’s routing table
can be redistributed.
8
Route Redistribution
Why configure redistribution?
Company mergers and different IGPs are used Company has different divisions with the network under separate control for business or political reasons Company has connections between business partners To allow multivendor interoperability (OSPF on non-Cisco, EIGRP on Cisco, for instance)
Configuring Redistribution
My best path to 192.100.10.0
is this way.
R2 and R3 are running both OSPF and EIGRP 192.168.10.0
R3 R1 EIGRP Routing Loop!
OSPF My best path to R2 192.100.10.0 is this way.
Incompatible routing information
Each routing protocol uses different metrics.
EIGRP uses slowest BW and cumulative Delay OSPF use cumulative BW Metrics cannot be translated exactly into a different protocol Path selection may not be optimal.
Potential Routing loops
– Depending on how redistribution is used, routers can send routing information received from one AS back into the AS. (Route Feedback)
Inconsistent convergence times:
Different routing protocols converge at different rates. These potential trouble spots can be avoided with careful planning and implementation.
Selecting the Best Route in a Redistribution Environment
Cisco routers use the following two parameters to select the best path:
Administrative distance:
Trustworthiness of the routing source Modifying the administrative distance to influence the route selection process is discussed later When using route redistribution, you might occasionally need to modify a protocol’s administrative distance so that it is preferred and to prevent routing loops. (later)
Routing metric:
Best path 11
Concepts of Redistribution
Multiple Routing Processes
Cisco routers support up to 30 dynamic routing processes on a single router. Most routing protocols allow an administrator to configure multiple processes of the same routing algorithm RIP and BGP are notable exceptions.
RTA#show running-config router ospf 24 !
network 10.2.0.0 0.0.255.255 area 0
Not usually
router ospf 46
recommended
!
network 192.168.2.0 0.0.0.255 area 2 router eigrp 53 network 172.16.0.0
network 172.17.0.0
!
router eigrp 141 network 10.0.0.0
network 192.168.3.0
Not usually recommended
Route Redistribution
Route redistribution
between routing protocols.
EIGRP - The process of exchanging routing information routing domain learns about networks in OSPF routing domain. OSPF routing domain learns about networks in domain. EIGRP routing Done by a
boundary router
which participates in both routing protocols.
Redistribution Concepts and Processes
I run both EIGRP and OSPF .
Router(config-router)# redistribute
from-protocol
[
process-id
]
Note: Other parameters may be required and will be discussed.
The
redistribution
command (“take routes from”) Configured on the boundary router. Participates in both routing protocols.
Independent of any one protocol Various complexities depending on the routing protocols and the options.
Redistributing from OSPF into EIGRP
Our Topology
EIGRP 1 OSPF 1 Boundary router R2-E-O is running: EIGRP for 172.30.0.0 subnets and 172.31.0.0 network OSPF for 172.16.0.0 subnets and 172.17.0.0 network 192.168.1.0 or 10.0.0.0 not currently included in either routing protocol (more on this later)
Redistribution into EIGRP
redistribute protocol [
process-id bw delay reliability load mtu
| ] [
as-number
match
{ ] [
metric internal
|
nssa-external
|
external 1
value
] [ route-map name ] |
external 2
}] [ tag tag- The syntax differs slightly depending on the routing protocol into which routes will be redistributed.
Redistribution into EIGRP
redistribute protocol [
process-id bw delay reliability load mtu
| ] [
as-number
match
{ ] [
metric internal
|
nssa-external
|
external 1
value
] [ route-map name ] |
external 2
}] [ tag tag-
protocol
- The source of routing information. Includes RIP, OSPF, EIGRP, IS-IS, BGP, connected, and static.
process-id
,
as-number
or ASN value.
- If redistributing a routing protocol that uses a process-id or ASN on the router global config command, use this parameter to refer to that process
metric
- A keyword after which follows the four metric components (bandwidth, delay, reliability, link load), plus the MTU associated with the route.
match
- If redistributing from OSPF, this keyword lets you match internal OSPF routes, external (by type), and NSSA external routes, essentially filtering which routes are redistributed.
tag
- Assigns a unitless integer value to the route, which can be later matched by other routers using a route-map.
route-map
- Apply the logic in the referenced route-map to filter routes, set metrics, and set route tags.
19
Redistribution into EIGRP
Current configurations
R1-E router eigrp 1 network 172.30.0.0
network 172.31.0.0
auto-summary R2-E-O router eigrp 1 network 172.30.0.0
auto-summary router ospf 1 network 172.16.0.0 0.0.0.3 area 0 R3-O router ospf 1 network 172.16.0.0 0.0.255.255 area 0 R4-O router ospf 1 network 172.16.0.0 0.0.255.255 area 0 network 172.17.0.0 0.0.255.255 area 0
What networks do I know about
Redistribution
learn about
into EIGRP
them?
What do you expect to see?
Directly Connected and any EIGRP networks – NO OSPF networks
R1-E# show ip route C 172.31.0.0/16 is directly connected, Loopback31 172.30.0.0/16 is variably subnetted, 6 subnets, 3 masks C 172.30.2.0/24 is directly connected, FastEthernet0/1 C 172.30.3.0/24 is directly connected, Loopback0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.0.0/16 is a summary, 00:02:41, Null0 C 172.30.1.0/24 is directly connected, FastEthernet0/0 C 172.30.4.0/24 is directly connected, Loopback1 R1-E#
Redistribution into EIGRP
What do you expect to see?
EIGRP and OSPF networks
R2-E-O# show ip route
What networks do I know about and how did I
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
them?
O 172.16.0.4/30 [110/845] via 172.16.0.1, 00:02:32, Serial0/1 C 172.16.0.0/30 is directly connected, Serial0/1 O 172.16.1.0/24 [110/782] via 172.16.0.1, 00:02:32, Serial0/1 O 172.16.2.0/24 [110/846] via 172.16.0.1, 00:02:32, Serial0/1 D 172.31.0.0/16 [90/20640000] via 172.30.0.1, 00:03:46, Serial0/0 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks D 172.30.2.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.3.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.1.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.4.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets C 10.0.0.0 is directly connected, FastEthernet0/1 C 192.168.1.0/24 is directly connected, FastEthernet0/0 R2-E-O#
Redistribution into EIGRP
What do you expect to see?
Only OSPF networks – NO EIGRP networks
R3-O# show ip route
What networks do I know about and how did I learn about them?
O 172.17.0.0/16 [110/65] via 172.16.0.6, 00:09:06, Serial0/2 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks C 172.16.0.4/30 is directly connected, Serial0/2 C 172.16.0.0/30 is directly connected, Serial0/1 C 172.16.1.0/24 is directly connected, FastEthernet0/0 O 172.16.2.0/24 [110/65] via 172.16.0.6, 00:09:06, Serial0/2 R3-O#
Redistribution into EIGRP
What do you expect to see?
Only OSPF networks – NO EIGRP networks
R4-O# show ip route
What networks do I know about and how did I learn about them?
C 172.17.0.0/16 is directly connected, FastEthernet0/1 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks C 172.16.0.4/30 is directly connected, Serial0/0 O 172.16.0.0/30 [110/128] via 172.16.0.5, 00:09:52, Serial0/0 O 172.16.1.0/24 [110/65] via 172.16.0.5, 00:09:52, Serial0/0 C 172.16.2.0/24 is directly connected, FastEthernet0/0 R4-0#
Hey! I don’t see any of the networks in the
Redistribution
happened?
into EIGRP
No change for R1-E!
No OSPF networks Let’s see what happened (or didn’t happen)…
R2-E-O(config)#
router eigrp 1
R2-E-O(config-router)#
redistribute ospf 1
R1-E#
show ip route
I will redistribute my OSPF learned networks (
and OSPF network command networks
) into EIGRP, telling my EIGRP neighbors about these networks
C 172.31.0.0/16 is directly connected, Loopback31 172.30.0.0/16 is variably subnetted, 6 subnets, 3 masks C 172.30.2.0/24 is directly connected, FastEthernet0/1 C 172.30.3.0/24 is directly connected, Loopback0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.0.0/16 is a summary, 00:02:41, Null0 C 172.30.1.0/24 is directly connected, FastEthernet0/0 C 172.30.4.0/24 is directly connected, Loopback1 R1-E#
Redistribution into EIGRP
Should R2’s routing table change?
No R2-E-O# show ip route O 172.17.0.0/16 [110/846] via 172.16.0.1, 00:02:32, Serial0/1 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks O 172.16.0.4/30 [110/845] via 172.16.0.1, 00:02:32, Serial0/1 C 172.16.0.0/30 is directly connected, Serial0/1 O 172.16.1.0/24 [110/782] via 172.16.0.1, 00:02:32, Serial0/1 O 172.16.2.0/24 [110/846] via 172.16.0.1, 00:02:32, Serial0/1 D 172.31.0.0/16 [90/20640000] via 172.30.0.1, 00:03:46, Serial0/0 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks D 172.30.2.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.3.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.1.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.4.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets C 10.0.0.0 is directly connected, FastEthernet0/1 C 192.168.1.0/24 is directly connected, FastEthernet0/0 R2-E-O#
26
Redistribution into EIGRP
R2-E-O(config)#
router eigrp 1
R2-E-O(config-router)#
redistribute ospf 1 R2-E-O# show ip eigrp top IP-EIGRP Topology Table for AS(1)/ID(192.168.1.1) P 172.30.2.0/24, 1 successors, FD is 20514560 via 172.30.0.1 (20514560/28160), Serial0/0 P 172.30.3.0/24, 1 successors, FD is 20640000 via 172.30.0.1 (20640000/128256), Serial0/0 P 172.30.0.0/30, 1 successors, FD is 20512000 via Connected, Serial0/0 P 172.31.0.0/16, 1 successors, FD is 20640000 via 172.30.0.1 (20640000/128256), Serial0/0 P 172.30.1.0/24, 1 successors, FD is 20514560 via 172.30.0.1 (20514560/28160), Serial0/0 P 172.30.4.0/24, 1 successors, FD is 20640000 via 172.30.0.1 (20640000/128256), Serial0/0
For now notice that there are no “OSPF networks” in R2’s topology table.
They are still in the routing table because R2 also runs OSPF, but this is an EIGRP command.
27
Redistribution into EIGRP
BW/DLY BW redistribute
protocol
[
process-id delay reliability load mtu
] |
as-number
] [metric
bw
default-metric
bw delay reliability load mtu
When redistributing into EIGRP from another routing protocol you must convert the other routing protocol’s metric (OSPF’s cost, bandwidth) into EIGRP’s metric (BW, DLY, Reliability and Load).
This metric, referred to as the
seed or default metric
, is defined during redistribution configuration. Three methods:
Metric
parameter with
redistribute
Sets the default for all
redistribute
command commands
Default-metric
command Sets the default for all
redistribute
commands
Route-map
Sets different metrics for routes learned from a single source
Redistribution into EIGRP
10000 100 255 1 EIGRP 1 OSPF 1 EIGRP 2 50000 500 255 1
router eigrp 1 network 172.20.0.0 redistribute ospf 1 redistribute eigrp 2 default-metric 10000 100 255 1 1500 redistribute rip metric 50000 500 255 1 1500
RIP default-metric command is used where the metric applied in the redistribute command.
parameter is not being metric parameter takes precedence over the default-metric command Note: The metric will give all redistributed networks the same starting metric.
This is known as the seed metric
Redistribution into EIGRP
1000 33 255 1 R2-E-O(config)# router eigrp 1 R2-E-O(config-router)# redistribute ospf 1 R2-E-O(config-router)# default-metric 1000 33 255 1 1500
BW DLY RLY Load MTU
OR R2-E-O(config)# router eigrp 1
BW DLY RLY Load MTU
R2-E-O(config-router)# redistribute ospf 1 metric 1000 33 255 1 1500 Note: MTU is NOT one of the EIGRP metrics (never has been, never will be) MTU is included because it is tracked through the path to find the smallest MTU.
Redistribution into EIGRP
R2-E-O# show ip eigrp top IP-EIGRP Topology Table for AS(1)/ID(192.168.1.1) P 172.16.0.4/30, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0) P 172.16.0.0/30, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0) P 172.16.1.0/24, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0) P 172.17.0.0/16, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0) P 172.16.2.0/24, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0)
New Entries EIGRP topology table lists the outgoing interface as " via redistributed " All the redistributed routes have the same feasible distance (FD) calculation ( 2568448 ), because all use the same component metrics per the configured
default-metric
command 31
Redistribution into EIGRP
R2-E-O# show ip eigrp top 172.16.0.0/30 IP-EIGRP (AS 1): Topology entry for 172.16.0.0/30 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2568448 Routing Descriptor Blocks: 0.0.0.0, from Redistributed, Send flag is 0x0 Composite metric is (2568448/0), Route is External Vector metric: Minimum bandwidth is 1000 Kbit Total delay is 330 microseconds
From
default-metric
command
Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 0 External data:
"( this system )", meaning that the router on which the command was issued (R2 in this case) redistributed the route.
Originating router is 192.168.1.1 (this system) AS number of route is 1 External protocol is OSPF , external metric is 0 Administrator tag is 0 (0x00000000)
32
Great! Now I see all the networks in the
Redistribution
EIGRP routes.
into EIGRP
R1-E# show ip route D EX D EX D EX D EX 172.17.0.0/16 [ 170/3080448 ] via 172.30.0.2, 00:01:50, Serial0/0 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks 172.16.0.4/30 [ 170/3080448 ] via 172.30.0.2, 00:01:50, Serial0/0 172.16.0.0/30 [ 170/3080448 ] via 172.30.0.2, 00:01:50, Serial0/0 172.16.1.0/24 [ 170/3080448 ] via 172.30.0.2, 00:01:50, Serial0/0 D EX 172.16.2.0/24 [ 170/3080448 ] via 172.30.0.2, 00:01:50, Serial0/0 C 172.31.0.0/16 is directly connected, Loopback31 EX: External Route (redistributed) 172.30.0.0/16 is variably subnetted, 6 subnets, 3 masks C 172.30.1.0/24 is directly connected, FastEthernet0/0 C 172.30.4.0/24 is directly connected, Loopback1
Redistribution into EIGRP
R2-E-O# show ip eigrp top P 172.16.0.0/30, 1 successors, FD is 2568448 via Redistributed ( 2568448 /0)
R2 redistributed into EIGRP the routes learned via OSPF and its own directly connected network 172.16.0.0/30.
But
not
192.168.1.0/24 and 10.0.0.0/8 This is because 172.16.0.0/30 is an OSPF enabled interface (network statement) Redistribute command, redistributes the following: All routes in the routing table learned by that routing protocol All connected routes of interfaces on which that routing protocol is enabled Otherwise must be redistributed another way (connected or static) – coming 34
Redistribution into EIGRP
What about the 10.0.0.0/24 network? How can I redistribute it into EIGRP?
R2-E-O(config)#
router ospf 1
R2-E-O(config-router)#
network 10.0.0.0 0.0.0.255 area 0
R2-E-O#
show ip route
10.0.0.0/24 is subnetted, 1 subnets C 10.0.0.0 is directly connected, FastEthernet0/1 No change to routing table Two ways to redistribute 10.0.0.0/24 network.
Redistribute Connected Add OSPF network command Also propagates 10.0.0.0/24 throughout OSPF domain
The 10.0.0.0 network is now included as
Redistribution
routes.
into EIGRP
R1-E# show ip route D EX 10.0.0.0 [170/3080448] via 172.30.0.2, 00:01:33, Serial0/0 R4-0# show ip route O 10.0.0.0 [110/129] via 172.16.0.5, 00:04:02, Serial0/0
10.0.0.0 is now redistributed into the EIGRP domain with the rest of the OSPF networks.
Redistribution into EIGRP
What about the 192.168.1.0 network? How can I redistribute it into EIGRP?
R2-E-O(config)#
router eigrp 1
R2-E-O(config-router)#
redistribute connected metric 1000 33 255 1 1500
R1-E#
show ip route
10.0.0.0/24 is subnetted, 1 subnets D EX 10.0.0.0 [170/
3080448
] via 172.30.0.2, 00:01:57, Serial0/0 D EX
192.168.1.0/24
[170/
3080448
] via 172.30.0.2, 00:01:57, Serial0/0 R1-E# 192.168.1.0/24 is redistributed into EIGRP as a connected network.
metric
option is not required for this command (default 0, but beyond the scope of this pres.) 192.168.1.0/24 is redistributed into the EIGRP domain using the default metric but it is NOT propagated throughout OSPF domain
Redistribution into EIGRP
R2: Currently router eigrp 1 network 172.30.0.0
auto-summary redistribute ospf 1 default-metric 1000 33 255 1 1500 redistribute connected 1000 33 255 1 1500 !
Where we left off…
router ospf 1 network 10.0.0.0 0.0.0.255 area 0 network 172.16.0.0 0.0.0.3 area 0
Redistributing from EIGRP into OSPF
Redistribution into OSPF
BW/DLY BW redistribute protocol {
metric-value
| [
process-id
transparent
}] [ |
as-number
] [
metric
metric-type type-
value
] [
match
{
internal
|
external 1
|
external 2 | nssa-external
}] [ tag tag-value ] [ route-map map-tag ] [
subnets
] Several similarities and differences to redistributing into EIGRP.
In this case we must convert the EIGRP metric to the Cisco OSPF metric of Bandwidth.
Redistribution into OSPF
redistribute protocol {
metric-value
| [
process-id
transparent
}] [ |
as-number
] [
metric
metric-type type-value ] [
match
{
internal
|
external 1
|
external 2 | nssa external
}] [ tag tag-value ] [ route-map map-tag ] [
subnets
]
Metric
- Defines the cost metric assigned to the route in the Type 5 (or Type 7 if NSSA) LSA. metric transparent when taking from another OSPF process, pass through the metric with the route.
metric-type {1 | 2} -
routes).
Defines the external metric type of 1 (E1 routes) or 2 (E2
Match -
If redistributing from OSPF, this keyword lets you match internal OSPF routes, external (by type), and NSSA external routes, essentially filtering which routes are redistributed.
Tag -
Assigns a unitless integer value to the route, which can be later matched by other routers using a route-map.
route-map -
Apply the logic in the referenced route-map to filter routes, set metrics, and set route tags.
Subnets
- Redistribute subnets of classful networks. Without this parameter, only routes for classful networks are redistributed. (This behavior is particular to the OSPF redistribute command.) 41
Redistribution into OSPF
BW/DLY BW=20 BW=20 redistribute protocol {
metric-value
| [
process-id
transparent
}] [ |
as-number
] [
metric
metric-type type-value ] [
match
{
internal
|
external 1
|
external 2 | nssa external
}] [ tag tag-value ] [ route-map map-tag ] [
subnets
] Defaults when redistributing into OSPF: When redistributing networks from all other sources the default metric is 20.
External metric type 2 (metric does not change throughout OSPF routing domain) Only redistributes routes of classful (Class A, B, and C) networks, and not for subnets
Redistribution into OSPF
R2: Currently router eigrp 1 network 172.30.0.0
auto-summary redistribute ospf 1 default-metric 1000 33 255 1 1500 redistribute connected 1000 33 255 1 1500 !
Where we left off…
router ospf 1 network 10.0.0.0 0.0.0.255 area 0 network 172.16.0.0 0.0.0.3 area 0
Redistribution into OSPF
What do you expect to see?
EIGRP and OSPF networks
R2-E-O# show ip route
What networks do I know about and how did I learn about
O 172.17.0.0/16 [110/846] via 172.16.0.1, 00:02:32, Serial0/1
them?
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks O 172.16.0.4/30 [110/845] via 172.16.0.1, 00:02:32, Serial0/1 C 172.16.0.0/30 is directly connected, Serial0/1 O 172.16.1.0/24 [110/782] via 172.16.0.1, 00:02:32, Serial0/1 O 172.16.2.0/24 [110/846] via 172.16.0.1, 00:02:32, Serial0/1 D 172.31.0.0/16 [90/20640000] via 172.30.0.1, 00:03:46, Serial0/0 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks D 172.30.2.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.3.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.1.0/24 [90/20514560] via 172.30.0.1, 01:22:36, Serial0/0 D 172.30.4.0/24 [90/20640000] via 172.30.0.1, 01:22:36, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets C 10.0.0.0 is directly connected, FastEthernet0/1 C 192.168.1.0/24 is directly connected, FastEthernet0/0 R2-E-O#
Redistribution into OSPF
R2-E-O# show ip ospf data OSPF Router with ID (192.168.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 172.16.1.1 172.16.1.1 85 0x80000005 0x006220 5 172.30.0.6 172.30.0.6 2000 0x80000006 0x006BB4 4 192.168.1.1 192.168.1.1 1117 0x80000003 0x009742 3 R2-E-O#
No External Type 5 LSAs No EIGRP networks being redistributed into OSPF 45
Redistribution into OSPF
R2-E-O(config)#
router ospf 1
R2-E-O(config-router)#
redistribute eigrp 1 % Only classful networks will be redistributed
R2-E-O(config-router)# R2-E-O#
show ip ospf data
Subnets will not be redistributed Supernets will also be redistributed (such as 173.0.0.0/8)
Redistribution into OSPF
Remember, routes are only Redistributed if they are in the Routing table
R2-E-O# show ip route O 172.17.0.0/16 [110/846] via 172.16.0.1, 00:03:56, Serial0/1 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks O 172.16.0.4/30 [110/845] via 172.16.0.1, 00:03:56, Serial0/1 C 172.16.0.0/30 is directly connected, Serial0/1 O 172.16.1.0/24 [110/782] via 172.16.0.1, 00:03:56, Serial0/1 O 172.16.2.0/24 [110/846] via 172.16.0.1, 00:03:56, Serial0/1 D 172.31.0.0/16 [90/20640000] via 172.30.0.1, 00:18:29, Serial0/0 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks D 172.30.2.0/24 [90/20514560] via 172.30.0.1, 01:37:19, Serial0/0 D 172.30.3.0/24 [90/20640000] via 172.30.0.1, 01:37:19, Serial0/0 C 172.30.0.0/30 is directly connected, Serial0/0 D 172.30.1.0/24 [90/20514560] via 172.30.0.1, 01:37:19, Serial0/0 D 172.30.4.0/24 [90/20640000] via 172.30.0.1, 01:37:19, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets C 10.0.0.0 is directly connected, FastEthernet0/1 C 192.168.1.0/24 is directly connected, FastEthernet0/0
47
Redistribution into OSPF
R3-O#
show ip route
I only see the class B 172.31.0.0/16 network in the EIGRP domain.
O 172.17.0.0/16 [110/65] via 172.16.0.6, 00:01:16, Serial0/2 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks C 172.16.0.4/30 is directly connected, Serial0/2 C 172.16.0.0/30 is directly connected, Serial0/1 C 172.16.1.0/24 is directly connected, FastEthernet0/0 O 172.16.2.0/24 [110/65] via 172.16.0.6, 00:01:16, Serial0/2 O E2 172.31.0.0/16 [110/20] via 172.16.0.2, 00:01:16, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/65] via 172.16.0.2, 00:01:17, Serial0/1 R3-O# Only the class B network 172.31.0.0/16 is redistributed into OSPF
Redistribution into OSPF
R3-O# show ip ospf data
External Type 5 LSA 49
Redistribution into OSPF
I will add the subnets option.
R2-E-O(config)#
router ospf 1
R2-E-O(config-router)#
redistribute eigrp 1 subnets
No warning message “Only classful networks will be redistributed”
Subnets
– Subnets are now included in the redistribution.
Redistribution into OSPF
R2-E-O# show ip ospf data Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 172.30.0.0 192.168.1.1 79 0x80000001 0x008EDE 0 172.30.1.0 192.168.1.1 79 0x80000001 0x0095D3 0 172.30.2.0 192.168.1.1 79 0x80000001 0x008ADD 0 172.30.3.0 192.168.1.1 79 0x80000001 0x007FE7 0 172.30.4.0 192.168.1.1 79 0x80000001 0x0074F1 0 172.31.0.0 192.168.1.1 220 0x80000001 0x0094D4 0 R2-E-O#
R2 now includes Type 5 LSAs for subnets 51
Redistribution into OSPF – E2
BW=20 R3-O#show ip route Now I see all O 172.17.0.0/16 [110/65] via 172.16.0.6, 00:13:41, Serial0/2 from the EIGRP domain.
External OSPF routes are E2 with a default cost of 20. O 172.16.2.0/24 [110/65] via 172.16.0.6, 00:13:41, Serial0/2 O E2 172.31.0.0/16 [110/20] via 172.16.0.2, 00:13:41, Serial0/1 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks O E2 172.30.2.0/24 [110/20] via 172.16.0.2, 00:00:12, Serial0/1 O E2 172.30.3.0/24 [110/20] via 172.16.0.2, 00:00:12, Serial0/1 O E2 172.30.0.0/30 [110/20] via 172.16.0.2, 00:00:12, Serial0/1 O E2 172.30.1.0/24 [110/20] via 172.16.0.2, 00:00:14, Serial0/1 O E2 172.30.4.0/24 [110/20] via 172.16.0.2, 00:00:14, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/65] via 172.16.0.2, 00:00:14, Serial0/1
Redistribution into OSPF
BW=20 BW=20 R4-0# show ip route C 172.17.0.0/16 is directly connected, FastEthernet0/1 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks External OSPF routes are E2 with a default cost of 20. C 172.16.0.4/30 is directly connected, Serial0/0 O 172.16.0.0/30 [110/128] via 172.16.0.5, 00:04:02, Serial0/0 cost, irrespective of the interior cost to reach that route. C 172.16.2.0/24 is directly connected, FastEthernet0/0 O E2 172.31.0.0/16 [110/20] via 172.16.0.5, 00:04:02, Serial0/0 172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks O E2 172.30.2.0/24 [110/20] via 172.16.0.5, 00:01:46, Serial0/0 O E2 172.30.3.0/24 [110/20] via 172.16.0.5, 00:01:46, Serial0/0 O E2 172.30.0.0/30 [110/20] via 172.16.0.5, 00:01:46, Serial0/0 O E2 172.30.1.0/24 [110/20] via 172.16.0.5, 00:01:46, Serial0/0 O E2 172.30.4.0/24 [110/20] via 172.16.0.5, 00:01:46, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/129] via 172.16.0.5, 00:04:04, Serial0/0
Redistribution into OSPF
R4-0# show ip ospf data Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 172.30.0.0 192.168.1.1 113 0x80000001 0x008EDE 0 172.30.1.0 192.168.1.1 113 0x80000001 0x0095D3 0 172.30.2.0 192.168.1.1 113 0x80000001 0x008ADD 0 172.30.3.0 192.168.1.1 113 0x80000001 0x007FE7 0 172.30.4.0 192.168.1.1 113 0x80000001 0x0074F1 0 172.31.0.0 192.168.1.1 254 0x80000001 0x0094D4 0 R4-0#
R4 now includes Type 5 LSAs for subnets 54
Redistribution into OSPF
R2-E-O(config)#
router ospf 1
R2-E-O(config-router)#
redistribute connected ?
metric Metric for redistributed routes metric-type OSPF/IS-IS exterior metric type for redistributed routes route-map Route map reference subnets Consider subnets for redistribution into OSPF tag Set tag for routes redistributed into OSPF
router ospf 1
R2-E-O(config-router)#
redistribute connected % Only classful networks will be redistributed
R2-E-O(config-router)#
redistribute connected subnets
Let’s redistribute the 192.168.1.0/24 network into OSPF as a connected network.
This is okay because 192.168.1.0/24 is a Class C network.
If it was a subnet then…
Redistribution into OSPF
R4-0#
show ip route
show ip ospf data
Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag
Summary so far…
1000 33 255 1 BW=20 BW=20 R2 summary: router eigrp 1 network 172.30.0.0
auto-summary redistribute ospf 1
OSPF learned networks are distributed into the EIGRP domain
default-metric 1000 33 255 1 1500
Use the metrics for BW DLY RLY Load
!
redistribute connected metric 1000 33 255 1 1500
Distribute any directly connected networks and use these metrics for BW DLY RLY Load
router ospf 1 So far… network 172.16.0.0 0.0.0.3 area 0 redistribute eigrp 1 subnets redistribute connected
EIGRP learned networks are distributed into the OSPF domain, default metric of 20 Distribute any directly connected networks and use default metric of 20
Redistribution into OSPF – E2
R3-O#show ip route O 172.17.0.0/16 [110/65] via 172.16.0.6, 00:13:41, Serial0/2 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks C 172.16.0.4/30 is directly connected, Serial0/2 C 172.16.0.0/30 is directly connected, Serial0/1 C 172.16.1.0/24 is directly connected, FastEthernet0/0 O 172.16.2.0/24 [110/65] via 172.16.0.6, 00:13:41, Serial0/2 O E2 172.31.0.0/16 [110/20] via 172.16.0.2, 00:13:41, Serial0/1 172.30.0.0/24 is subnetted, 4 subnets O E2 172.30.2.0 [110/20]
External OSPF routes are E2 with a
O E2 172.30.3.0 [110/20]
default cost of
20
.
O E2 172.30.1.0 [110/20] O E2 172.30.4.0 [110/20] via 172.16.0.2, 00:11:25, Serial0/1 metric-type 2 -
The cost of a
via 172.16.0.2, 00:11:25, Serial0/1 type 2
route is always the external cost,
10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/65] via 172.16.0.2, 00:13:43, Serial0/1 O E2 192.168.1.0/24 [110/20]
that route.
58
Redistribution into OSPF – E2
R4-0#show ip route C 172.17.0.0/16 is directly connected, FastEthernet0/1 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks C 172.16.0.4/30 is directly connected, Serial0/0 O 172.16.0.0/30 [110/128] via 172.16.0.5, 00:14:05, Serial0/0 O 172.16.1.0/24 [110/65] via 172.16.0.5, 00:14:05, Serial0/0 C 172.16.2.0/24 is directly connected, FastEthernet0/0 O E2 172.31.0.0/16 [110/20] via 172.16.0.5, 00:14:05, Serial0/0 172.30.0.0/24 is subnetted, 4 subnets O E2 172.30.2.0 [110/20]
External OSPF routes are E2 with a
O E2 172.30.3.0 [110/20] O E2 172.30.1.0 [110/20] via 172.16.0.5, 00:11:49, Serial0/0 O E2 172.30.4.0 [110/20] via 172.16.0.5, 00:11:49, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/129] via 172.16.0.5, 00:14:07, Serial0/0 O E2 192.168.1.0/24 [110/20 ] via 172.16.0.5, 00:07:54, Serial0/0
59
Redistribution into OSPF modifying the metric
R2-E-O(config)#router ospf 1 R2-E-O(config-router)#redistribute eigrp 1 subnets metric 100 R2-E-O(config-router)#redistribute connected R4-0#show ip route
192.168.1.0/24 still has a cost of 20. Why?
It was redistributed with the redistribute connected command without the
metric 100
parameter. <
redistribute connected metric 100
> 60
Redistribution into OSPF – E1
R2-E-O(config)# router ospf 1 R2-E-O(config-router)# redistribute eigrp 1 subnets metric-type 1 R2-E-O# show run router ospf 1 log-adjacency-changes
Notice that the previous
metric 100
parameter is still included!
redistribute connected redistribute eigrp 1 metric 100 metric-type 1 subnets network 10.0.0.0 0.0.0.255 area 0 network 172.16.0.0 0.0.0.3 area 0
metric-type {1 | 2} -
routes). Defines the external metric type of 1 (E1 routes) or 2 (E2
metric-type 1 -
A
type 1
cost is the addition of the external cost and the internal cost used to reach that route.
metric-type 2 -
The cost of a
type 2
the interior cost to reach that route. route is always the external cost, irrespective of 61
Redistribution into OSPF
R3-O#show ip route O E1 172.31.0.0/16 [110/164] via 172.16.0.2, 00:00:23, Serial0/1 172.30.0.0/24 is subnetted, 4 subnets O E1 172.30.2.0 [110/164] O E1 172.30.3.0 [110/164] via 172.16.0.2, 00:00:24, Serial0/1 via 172.16.0.2, 00:00:24, Serial0/1 O E1 172.30.1.0 [110/164] O E1 172.30.4.0 [110/164] via 172.16.0.2, 00:00:24, Serial0/1 via 172.16.0.2, 00:00:24, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets O 10.0.0.0 [110/65] via 172.16.0.2, 00:21:45, Serial0/1 O E2 192.168.1.0/24 [110/20] via 172.16.0.2, 00:15:32, Serial0/1
E1 routes, seed metric of 100 plus internal cost.
192.168.1.0/24 still has a cost of 20. It was redistributed with the redistribute connected command without the
metric-type 1
parameter,
E2
is the default. <
redistribute connected metric 100 metric-type 1
> 62
More Redistribution Examples
63
Same Protocol Stack
You can only redistribute routes from routing protocols that support the same protocol stack. IPv4 to IPv4 IPv6 to IPv6 64
RIPv2 and OSPF Example
Routing tables prior to redistribution 65
X
The
passive-interface
command is configured for interface serial 0/0/2 to prevent RIPv2 from sending route advertisements out that interface. OSPF is configured on interface serial 0/0/2. 66
The goal of redistribution in this network is for all routers to recognize all routes within the company.
RIPv2 is redistributed into the OSPF process, and the metric is set using the
redistribute
command (help prevent routing loops - later). A metric value of
300
is selected because it is a worse metric than any belonging to a native OSPF route.
Routes from OSPF process 1 are redistributed into the RIPv2 process with a metric of
5
. A value of 5 is chosen because it is higher than any metric in the RIP network. 67
R 10.0.0.8/30 O E2 10.0.0.0/30
There is complete reachability; however, Routers A and C now have many more routes to keep track of than before. They also will be affected by any topology changes in the other routing domain. 68
For RIPv2 on Router A, the summarization command is configured on the interface connecting to Router B, interface S0/0/0. Interface S0/0/0 advertises the summary address instead of the individual subnets. 10.0.0.0 255.252.0.0 summarizes the four subnets on Router A (including the 10.0.0.0/30 subnet).
69
For OSPF, summarization must be configured on an area border router (ABR) or an ASBR. Router C summarization command is configured under the OSPF process on Router C. 10.8.0.0 255.252.0.0 summarizes the four subnets on Router C. 70
Redistribution Techniques and Issues
71
Seed Metric
router ospf 1 network 172.20.0.0 redistribute rip default-metric 501 or redistribute rip metric 501
501 Largest metric is 500 OSPF1
501
RIP When redistributing information, the seed metric should be set to a value larger than the largest metric within the receiving autonomous system (aka the largest native metric).
This will help prevent suboptimal routing and routing loops.
The
default seed metric
value for routes that are redistributed into each IP routing protocol. A metric of
infinity
tells the router that the route is unreachable and, therefore, should not be advertised. When redistributing routes into
RIP, IGRP, and EIGRP
, you must specify a seed metric, or the redistributed routes will not be advertised.
For
OSPF
, the redistributed routes have a
default type 2 (E2) metric of 20,
(except for redistributed BGP routes, which have a default type 2 metric of 1 ) 73
One-Point Redistribution
One-point redistribution has only one router redistributing between two routing protocols.
A one way redistribution issue that could occur… 74
R2 and R3
are both running OSPF and EIGRP Only
R2
is redistributing between OSPF en EIGRP
R1
has an External Route 10.0.0.0 that it is redistributing into its AS.
R1
is propagating this route to both R2 and R3.
10.0.0.0 via R1 has AD 170 (EX EIGRP) 10.0.0.0 via R2 has AD 110 (OSPF) So, I will choose (include in my routing table) the path via R2 (OSPF)
R3
receives routing update information for the external route 10.0.0.0. directly from:
R1 via EIGRP (AD = 170) R2 via OSPF (AD = 110)
Because the
AD of OSPF (110)
is lower than AD of external EIGRP routes (170),
R3 selects the OSPF route
. Suboptimal routing Instead of sending packets directly from router R3 to router R1, router R3 prefers the path via router R2, resulting in suboptimal routing.
Solution: R2 should redistribute EIGRP route into OSPF with an AD of 115.
We will see how to do this later.
75
Multipoint redistribution
Multipoint redistribution has two separate routers running both routing protocols. Two possibilities exist: Multipoint one-way redistribution Multipoint two-way redistribution Likely to introduce potential routing loops 76
10.0.0.0 via R1 has AD 170 (EX EIGRP) 10.0.0.0 via R3 has AD 110 (OSPF) 10.0.0.0 via R1 has AD 170 (EX EIGRP) 10.0.0.0 via R2 has AD 110 (OSPF) So, I will choose (include in my routing table) the path via R2 (OSPF)
R1
(EIGRP) is announcing routes, including the external route, to R2 and R3.
R2 and R3
are both running two routing protocols (EIGRP and OSPF) and redistributing EIGRP into OSPF. Therefore,
R2 and R3
receive routing update information for the external route 10.0.0.0: via EIGRP from router R1 and via OSPF (R2 from R3, and R3 from R2). The AD of OSPF (110) is lower than AD of external EIGRP (170): So R2 selects the OSPF route instead of sending packets directly to R1 R2 prefers the path via router R3 Routing Loop!
77
10.0.0.0 via R1 has AD 170 (EX EIGRP) 10.0.0.0 via R3 has AD 110 (OSPF) So, I will choose (include in my routing table) the path via R3 (OSPF) 10.0.0.0 via R1 has AD 170 (EX EIGRP) 10.0.0.0 via R2 has AD 110 (OSPF) So, I will choose (include in my routing table) the path via R2 (OSPF) To prevent routing loops in multipoint redistribution scenario the following recommendations should be considered:
Tag routes
in redistribution points and filter based on these tags when redistributing (later)
Modify the Administrative Distance
Use
default routes
of redistributed routes (later) to avoid having to do two-way redistribution 78
A multi-way multipoint redistribution issue The best path between R1 and R4 is via R3 But during redistribution from routing protocol B to routing protocol A, the metric is lost Domain A doesn’t know about metrics in Domain B R1 will send packets toward router R4 via router R2 (its best path outside its domain) Resulting in suboptimal routing. 79
Modifying Administrative Distance
80
AD
The administrative distance affects only the choice of path for identical IP routes.
In other words, routes that have identical prefix and mask.
Routes with a distance of 255 are not installed in the routing table .
81
Router(config-router)# distance administrative-distance
wildcard-mask
[
ip-standard- list
] [
ip-extended-list
[ ]]
address
This command can be used for all protocols.
There are additional options for each routing protocol.
82
distance eigrp 80 130
Sets the administrative distance for internal EIGRP routes to 80 and for external EIGRP routes to 130.
distance 90 192.168.7.0 0.0.0.255
Sets the administrative distance to 90 for all routes learned from routers on the Class C network 192.168.7.0
distance 120 172.16.1.3 0.0.0.0
Sets the administrative distance to 120 for all routes from the router with the address 172.16.1.3.
83
distance ospf external 100 inter-area 100 intra-area 100
Sets the administrative distance for external, inter-area, and intra-area OSPF routes to 100 (default values are 110).
distance 90 10.0.0.0 0.0.0.255, distance 110 10.11.0.0 0.0.0.255
, and
distance 130 10.11.12.0 0.0.0.255
Sets the administrative distance to 90, 110, and 130 respectively, for all routes learned from routers with specific addresses Routes from a router with address 10.10.0.1 will have an AD of 90 Routes from a router with address 10.11.12.1 will have an AD of 130.
84
R1 and R2 OSPF (AD 110) is by default considered more believable than RIPv2 (120) If R1 learns about network 10.3.3.0: via R2 (OSPF) Via R3 (RIPv2) The OSPF route is used because OSPF has a lower administrative distance than RIPv2, even though the path via OSPF might be the longer (worse) path.
AD = 110
Preferred
AD = 120 10.3.3.0/24
Example
85
Example
R1 and R2 Note: RIPv2 routes redistributed into OSPF have an OSPF seed metric of 10,000 (higher than any other OSPF route). This does
not
prevent our previous problem Makes these routes less preferred than native OSPF routes Protects against route feedback.
Prevents R1 from choosing R2 for OSPF routes it learns from internal OSPF routers.
The
redistribute
command also sets the metric type to 1 (external type 1) so that the route metrics continue to accrue. The routers also redistribute subnet information.
AD = 110 Metric = 10,000 Metric = 5 AD = 120 10.3.3.0/24 Metric = 10,000
86
Example
The OSPF routes redistributed into RIPv2 have a RIP seed metric of five hops to also protect against route feedback. R1 and R2
AD = 110 Metric = 10,000 Metric = 5 Metric = 10,000 AD = 120 10.3.3.0/24
87
My best path to all RIP networks is via R1 because OSPF (110) is better than RIP (120).
R2, receives information about the RIPv2 domain routes (also called the native RIPv2 routes) from both OSPF and RIPv2. R2 prefers the OSPF routes because OSPF has a lower administrative distance Therefore, none of the RIPv2 routes appears in R2’s routing table. All routes are via OSPF or directly connected.
88
Solution: Modifying the AD
You can change the administrative distance of the redistributed RIPv2 routes to ensure that the boundary routers select the native RIPv2 routes. The
distance
command on R1 and R2 changes the administrative distance of the OSPF routes to the networks that match access list 64 to
125
(from 110).
Access list 64 is used to match all the native RIPv2 routes. 89
R1 and R2
are assign an
AD of 125
to routes listed in access list
64
(routes learned from OSPF).
R1 and R2 prefer the native RIPv2 routes (AD 120) over the redistributed OSPF routes (AD 125) in their routing tables.
R1 will put the 10.200.200.34 network in its routing table as a RIP route (AD 120) instead of the OSPF (AD 125) route it learned via R2.
AD = 110 Metric = 10,000 AD = 125 Metric = 5 AD = 120
Preferred
Metric = 10,000 AD = 125
90
My best path to all RIP networks is via R4 because RIP (120) is better than redistributed RIP (125).
However, some routing information is lost with this configuration. For example, depending on the actual bandwidths, the OSPF path might have been better for the 10.3.1.0 network; it might have made sense not to include 10.3.1.0 in the access list for R2. 91
Verifying Redistribution
The best way to verify redistribution operation is as follows: Know your network topology, particularly where redundant routes exist.
Study the routing tables on a variety of routers in the internetwork using the
show ip route
Perform a trace using the
traceroute
on some of the routes that go across the autonomous systems to verify that the shortest path is being used for routing. 92
More on OSPF and External Routes
Determining the Next-hop for Type 2 External Routes
Intra-area
172.30.26.0/23
Review later slides for explanation
Metric = 20 LSA 5 Metric = 20 Best path
94
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23
LSA 4: I am ABR R3, I can reach ASBR R1 and my cost to the ASBR is 1.
Best path
LSA 4: I am ABR R4, I can reach ASBR R1 and my cost to the ASBR is 64.
Metric = 20 R5# show ip route O E2 172.30.26.0/23 [110/20] via 172.16.35.3, 05:48:42, Serial0/0
Review later slides for explanation 95
Comparing E1 and E2
EIGRP
ASBR1
OSPF E2 metric=10
ASBR2
E2 metric=20 The benefits of the different external route types apply mostly to when multiple ASBRs advertise the same subnet. Two ASBRs, ASBR1 and ASBR2, between OSPF and another routing domain. Goal is to always send traffic through ASBR1.
Configuration: Use E2 routes Set the metric for ASBR1's redistributed routes to a lower metric than ASBR2. Routers ignore the internal metrics when calculating the E2 metrics, so every router will choose ASBR1 as the better ASBR. 96
Comparing E1 and E2
EIGRP
ASBR1
E1 OSPF
ASBR2
E1 Goal is to: Balance the traffic Make each router pick the closest ASBR Configuration: Use E1 routes Routers closer to each ASBR choosing best routes based on the lower OSPF costs.
97
Comparing E1 and E2
EIGRP
ASBR1
E1 OSPF
ASBR2
E2 Note: OSPF routers will always prefers E1 routes over E2 routes for the same networks. 98
FYI: More on OSPF and External Routes
Redistribution into OSPF
EIGRP Area 0 OSPF Area 1
New Topology 100
Redistribution into OSPF
redistribute protocol {
metric-value
| [
process-id
transparent
}] [ |
as-number
] [
metric
metric-type type-value ] [
match
{
internal
|
external 1
|
external 2 | nssa external
}] [ tag tag-value ] [ route-map map-tag ] [
subnets
] Default if no
metric
configuration exists Cost 1 for routes learned from BGP Cost 20 for all other route sources
default-metric
cost OSPF subcommand Setting the default for all
redistribute
commands
metric
cost parameters on the
redistribute
Setting the metric for one route source command Metric
transparent
route source parameters on the
redistribute
command When taking routes from another OSPF process, using the metrics used by that Use the
route-map
parameter on the
redistribute
command Setting different metrics for routes learned from a single source 101
Redistribution into OSPF
Router that performs redistribution becomes ASBR (Autonomous System Border Router).
Injects external routes into OSPF creating a Type 5 LSA for each network/subnet . Type 5 LSA includes:
LSID:
the subnet number
Mask:
The subnet mask
Advertising router: Metric:
The RID of the ASBR injecting the route The metric as set by the ASBR
External Metric Type:
The external metric type, either 1 or 2 102
Redistribution into OSPF
ASBR floods Type 5 LSAs throughout area. If ABR is: Normal (non-stubby) areas: Flood Type 5 LSAs into area Stub and Totally Stubby areas: No Type 5 LSAs flooded Default route injected by ABR
LSA 5
103
Redistributing External Type 2 Routes
Redistribution into OSPF
172.30.26.0/23 LSA 5 Metric = 20 Metric = 20
E2 route’s metric is simply the metric in the Type 5 LSA.
Default = 20
metric
parameter R4 has two routes to 172.30.26.0/23: Via R1 Via R8 To avoid loops, OSPF routers use two tiebreaker systems to allow a router to choose a best external route. Router in question resides in the same area as the ASBR (intra-area) Router in question resides in a different area (interarea) than the ASBR 105
Determining the Next-hop for Type 2 External Routes -
Intra-area
172.30.26.0/23 Metric = 20 LSA 5 Metric = 20
Router has multiple routes for same E2 destination network: Selects the best route based on the lowest cost to reach any ASBR(s) that advertised the lowest E2 metric. R4: Both routes use metric 20 in this case, so the routes tie. Tiebreaker: 1.
Find the advertising ASBR(s) as listed in the Type 5 LSA(s) 2.
3.
4.
Using the intra-area LSDB topology calculate the best route to reach the ASBR(s). (This is the route that will be entered into the routing table.) This determines the outgoing interface and next hop based address to to reach the ASBR The route's metric is unchanged in the routing table as listed in theType 5 LSA 106
Determining the Next-hop for Type 2 External Routes
Intra-area
172.30.26.0/23 Metric = 20 LSA 5 Metric = 20 Best path
1.
2.
3.
4.
R4 looks in the Type 5 LSA, and sees RID 1.1.1.1 (R1) is the advertising ASBR.
R4 then looks at its area 0 LSDB entries, including the Type 1 LSA for RID 1.1.1.1, and calculates all possible area 0 routes to reach 1.1.1.1. R4's best route to reach RID 1.1.1.1 happens to be through its S0/0/0 interface, to next-hop RD1 (172.16.14.1), so R4's route to 172.16.26.0/23 uses these details.
The route lists metric 20, as listed in the Type 5 LSA. 107
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23 Metric = 20 LSA 5 Metric = 20
When router is in a different area same issues remain.
Different tiebreaker to reach ASBR.
Calculation requires more information that previous Intra-area example.
To calculate their best route to reach the ASBR, a router in another area:
Adds the cost to reach an ABR between the areas
Plus that ABR's cost to reach the ASBR
108
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23 1 Best path 64 64 64
R5 has two possible routes to reach ASBR: Via R3 Via R4 Although the metric is 20, R5 will use the cost to the ABR PLUS the ABR’s cost to the ASBR to determine the best path.
Via R3:
64 + 1 = 65
Via R4:
64 + 64 = 128
R5 chooses the route via R3 because it is a better path (65).
The router’s process for doing this is: 1.
Calculate the cost to reach the ABR, based on the area's topology database 2.
Add the cost from the ABR to the ASBR, as listed in a
Type 4 LSA
Let’s talk about that Type 4 LSA!
109
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23
LSA 4: I am ABR R3, I can reach ASBR R1 and my cost to the ASBR is 1.
LSA 4: I am ABR R4, I can reach ASBR R1 and my cost to the ASBR is 64.
LSA 4
The following slides provide additional information on LSA 4s if you are interested... Otherwise The End 110
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23
LSA 4: I am ABR R3, I can reach ASBR R1 and my cost to the ASBR is 1.
LSA 4: I am ABR R4, I can reach ASBR R1 and my cost to the ASBR is 64.
LSA 4
Type 4 Summary ASBR LSA: RID of the ASBR RID of the ABR that created and flooded the LSA 4 ABR's cost to reach the ASBR ABRs create Type 4 LSAs after receiving an external Type 5 LSA from an ASBR. ABR forwards a Type 5 LSA into an area ABR looks at the RID of the ASBR that created the Type 5 LSA.. ABR creates a Type 4 LSA listing that ASBR, and the cost to reach that ASBR, flooding that LSA into the neighboring areas. 111
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23
LSA 4: I am ABR R3, I can reach ASBR R1 and my cost to the ASBR is 1.
Best path
LSA 4: I am ABR R4, I can reach ASBR R1 and my cost to the ASBR is 64.
LSA 4
ABR R3 creates and floods Type 4 Summary ASBR LSA into area 1. ASBR 1.1.1.1 ( ASBR).
R1
), ABR 3.3.3.3 (
R3
), and
cost 1
(R3's cost to reach ABR R4 creates and floods Type 4 Summary ASBR LSA into area 1. ASBR 1.1.1.1 (
R1
), ABR 4.4.4.4 (
R4
), and lists
cost 64
(R4's cost to reach ASBR). When R5 finds two routes for subnet 172.30.26.0/23, and finds both have a
metric of 20
Break the tie. For each route: Add intra-area cost to reach the ABR PLUS the ABR's cost to reach the ASBR (as listed in the Type 4 LSA). R5 determines best route is through R3 has the lower cost (65). 112
Determining the Next-hop for Type 2 External Routes
Interarea
172.30.26.0/23
LSA 4: I am ABR R3, I can reach ASBR R1 and my cost to the ASBR is 1.
Best path
LSA 4: I am ABR R4, I can reach ASBR R1 and my cost to the ASBR is 64.
R5# show ip route O E2 172.30.26.0/23 [110/20] via 172.16.35.3, 05:48:42, Serial0/0
113
CIS 185 CCNP ROUTE Ch. 4 Manipulating Routing Updates
Rick Graziani Cabrillo College [email protected]