VRRP and Network Technologies
VRRP runs over many LAN media types such as Ethernet, FDDI, Token Ring, and the ATM LAN-emulated versions of Ethernet and Token Ring. VRRP uses both a MAC address and a unique IP address and its behavior is therefore not totally independent of the media on which it runs. Most of this book describes VRRP in the context of the most popular LAN media, Ethernet. This chapter takes a closer look at how VRRP works over other LAN media such as FDDI, Token Ring, and ATM LAN emulation and considers implementation-specific issues related to these media. Before looking at non-Ethernet media, we begin with a brief overview of VRRP over Ethernet.
5.1 VRRP over Ethernet
Over Ethernet, VRRP routers use a common MAC address of the format 00:00:5E: 00:01:XX. The first three octets are derived from the IANA's OUI. The next two octets (00:01) indicate the address block assigned to the VRRP protocol by IANA. The last octet is the VRID or the VRRP virtual router or group identifier, which provides for 255 virtual routers in a network.
Figure 5-1 shows two physical routers, R1 and R2. R1 has IP address 192.32. 15.1 / MAC of 00:00:A2:0B:00:01, backed up by R2, which has IP address 192.32. 15.2/ MAC of 00:00:A2:BE:D0:03. R1 is backing up R2. Therefore, there are two virtual routers in this picture: virtual router V1 with IP address IP(V1)=22.214.171.124, VRID of 37, and virtual router V2 with IP address IP(V2)=126.96.36.199, VRID of 73. Virtual router V1 has a VRRP MAC address of MAC(V1)=00:00:5E:00:01:25 (VRID 37, Hex 25), and virtual router V2 has a VRRP MAC address of MAC(V2)= 00:00:5E:00:01: 49 (VRID 73, Hex 49). Therefore, routers R1 and R2 will listen for MAC address of V1 and V2 in addition to their own MAC addresses. Host H1 is configured to use the default router of IP(V1); host H2 is configured to use the default router of IP(V2).
Figure 5-1. VRRP over Ethernet
R1 will be the master for virtual router V1 because it is the IP address owner. It will periodically send VRRP advertisements with the source MAC address set to its VRRP address and the destination set to the Ethernet multicast MAC address 01:00: 5E:00:00:12. The backup for virtual router V1, in this case R2, will listen passively to the advertisements by registering for the multicast MAC address and implement its state machine, as explained in Chapter 3 and 4. If R1 fails, R2 will take over as the master for V1 and act as the active router for V1.
Host H1, when it initializes, sends out an ARP request for its configured router IP address, 188.8.131.52. R1, which is the master for V1, will respond with the MAC address of V1, the VRRP MAC(V1). Host H1 will update its ARP cache with this MAC(V1). When host H1 wants to talk to a host H3 on another subnet (not shown in the figure), it will send a packet with the source MAC address set to MAC(H1), the destination MAC address set to MAC(V1), the source IP address of IP(H1), and the destination IP address of IP(H3). Router R1, which is currently the master for V1, will now receive this packet and forward it to its other interface to be eventually forwarded to H3. If R1 fails, host H1 need not make any changes; the packet will simply be forwarded to H3 via R2 completely transparent to H1.