Information Technology Reference
In-Depth Information
This step is service-affecting during the changeover for any prefixes that have R4 as an IGP
next hop. For example, the prefix 192.168.201.0/24 is originated by R6. That prefix is
deleted from the BGP RIB on R4 when the peer group membership is changed from a
regular iBGP session to a client-RR session. When traffic destined for 192.168.201.0/24
reaches R4, such traffic is dropped.
To avoid traffic loss, the following three approaches are available:
•
Shutting down the links between R4 and other core routers so that R4 is removed from
the forwarding paths for all the prefixes. This is probably the simplest method but
typically isn't recommended because BGP sessions cannot form when the links are
down.
•
Increasing IGP metrics from other core routers to R4 so that traffic entering the right
POP uses R5. Note that if IGP metrics between R4 and access routers in the same POP
are not changed, R4 may still be used as an IGP next hop for traffic leaving the POP. If
symmetric forwarding is desired, IGP metrics for all links of R4 should be increased.
•
Building additional BGP sessions between R4 and R6 and between R4 and R7 using
physical interface addresses or other loopback addresses. These temporary sessions
allow R4 to maintain all the routing information during the migration so that R4 can
share the traffic entering and leaving the POP.
The traffic loss is short and temporary during the configuration change and until new
routing information is learned.
NOTE
This case study chooses to move R4 out of the forwarding paths for 192.168.201.0/24 and
192.168.202.0/24 by increasing the IS-IS link metrics between R1 and R4 and between R2
and R4. When R4 learns these two prefixes after the peer group changes, R4 can be put back
in the forwarding paths by removing the metric changes.
Clients may receive additional paths for a prefix because the clients receive the prefix
directly from the peer that announces the prefix. They also receive the prefix from R4.
Example 8-14 shows the BGP RIB on R6 after the peer group membership change on R4.
There are two paths for each prefix that is not originated locally. The additional path is
reflected from R4.
Example 8-14
BGP RIB on R6 as a Client of R4
R6#show ip bgp
BGP table version is 5, local router ID is 192.168.100.6
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
