Information Technology Reference
In-Depth Information
Table 10-6 shows the loopback addresses used in Figure 10-24.
Table 10-6
Loopback Addresses Used in Figure 10-24
Router
Loopback Address
PE1
192.168.100.2
ASBR2
192.168.100.9
ASBR4
192.168.100.6
PE1's loopback address is part of the IGP in AS 100 and is distributed via LDP, L1, and L2,
to ASBR1. ASBR1 then advertises the address plus a new label, L3, to ASBR2 via eBGP,
with ASBR1 as the BGP next hop.
Example 10-31 shows the BGP labels on ASBR2. To reach PE1 (192.168.100.2), an out
label of 18 (L3) is used, and ASBR2 assigns a local label of 19 (L4). The BGP next hop for
the prefix is ASBR1 (192.168.59.5).
Example 10-31
BGP Labels on ASBR2
ASBR2#show ip bgp label
Network Next Hop In Label/Out Label
192.168.100.2/32 192.168.59.5 19/18
192.168.100.3/32 192.168.59.5 20/19
192.168.100.4/32 192.168.100.10 21/21
192.168.100.7/32 192.168.100.10 22/22
ASBR2 advertises the prefix with a label, L4, via iBGP to ASBR3 by setting itself as
the BGP next hop. Also, ASBR2 advertises via LDP an IGP label, L5, to reach itself
(192.168.100.9). PE1's address and label (L4) are now in ASBR3's BGP RIB. In turn,
ASBR3 advertises the prefix with a new label, L6, to ASBR4.
Example 10-32 shows the BGP labels on ASBR3. The value of L6, the in label, is 19. The
BGP next hop is ASBR2 (192.168.100.9). Now ASBR4 (192.168.100.6) can set itself as
the BGP next hop (using
next-hop-self
) and advertise the prefix with another label, L7, via
iBGP. An IGP label, L8, is also generated and advertised in LDP.
Example 10-32
BGP Labels on ASBR3
ASBR3#show ip bgp label
Network Next Hop In Label/Out Label
192.168.100.2/32 192.168.100.9 19/19
192.168.100.3/32 192.168.100.9 20/20
192.168.100.4/32 192.168.106.6 21/18
192.168.100.7/32 192.168.106.6 22/19
