Prefix Summarization

Load isis.inter.area.cfg

#IOS-XE
config replace flash:isis.inter.area.cfg

#IOS-XR
configure
load bootflash:isis.inter.area.cfg
commit replace
y

The topology has been changed so that there are three ISIS areas now.

• Configure R3 and R4 to summarize R1’s loopback as 1.1.1.0/24 into Level 2.

• Configure R4 to summarize R5’s loopback as 5.5.5.0/24 into Level 1. No other prefixes should be leaked into Level 1.

• Configure XR1 to summarize XR2’s loopback as 20.20.20.0/24 into Level 2.

• Configure XR2 to redistribute a new prefix, 20.20.200.20/32 into ISIS and summarize it as 20.20.200.0/24

Answer

#R3
router isis
 summary-address 1.1.1.0 255.255.255.0 level-2

#R4
ip prefix-list LEVEL2_TO_LEVEL1 permit 5.5.5.5/32
route-map LEVEL2_TO_LEVEL1
 match ip address prefix LEVEL2_TO_LEVEL1
!
router isis
 summary-address 1.1.1.0 255.255.255.0 level-2
 summary-address 5.5.5.0 255.255.255.0 level-1
 redistribute isis ip level-2 into level-1 route-map LEVEL2_TO_LEVEL1

#XR1
router isis 1
 add ipv4
  summary-prefix 20.20.20.0/24 level 2

#XR2
int lo20
 ip add 20.20.200.20/32
!
route-policy LO20
 if destination in ( 20.20.200.20/32 ) then pass endif
end-policy
!
router isis 1
 add ipv4
  redistribute connected route-policy LO20 level-1
  summary-prefix 20.20.200.0/24

Explanation

In ISIS, only L1/L2 routers can do inter-area summarization. This is similar to OSPF’s ABR doing summarization in type 3 LSAs. Like OSPF ASBRs, any ISIS router can be an ASBR and do external prefix summarization. But unlike OSPF, ISIS uses the same summary command for summarizing both inter-area and external prefixes.

When summarizing level 1 prefixes as they are advertised into level 2, you use the command summary-address <prefix> <mask> level-2. Optionally, you can leave the level-2 keyword out, as level 2 is the default.

When summarizing level 2 prefixes as they are advertised into level 1, you must first ensure that the routes are being leaked into level 1 in the first place. Then you use the same command but specify level-1. The level is always the level you are advertising the prefixes into.

When summarizing redistributed routes into ISIS, you also use the same exact summary-address/summary-prefix command. We must specify level-1 if we want to redistribute into level-1. By default, on both IOS-XR and IOS-XE, redistribution is only into level-2.

When adding a summary route, just like any other routing protocol, a null0 route is added to the RIB. In ISIS the AD for the summary route is still 115, unlike EIGRP which uses a lower AD for the summary route.

Verification

Verify that R1 and R2 know only 5.5.5.0/24 and no other L2 prefixes.

Verify that R5 knows 1.1.1.0/24, 20.20.20.0/24, and 20.20.200.0/24 instead of the specific /32 routes:

We can examine R3 and R4’s L2 LSP, and should see that both are injecting the 1.1.1.0/24 summary:

We can also see the summary route on any router preforming summarization: